Document that the WebNode class and its subclasses must not have any virtual methods.
Document that the WebNode class and its subclasses must not have any virtual methods.

This requirement allows code using classes in this heirarchy to safely
static_cast to the appropriate subclass based on the actual type of the wrapped
node.

BUG=278295
TEST=none
R=jamesr@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23453016

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@156933 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs